I recently built a 2 inch piston valve with 1 inch porting. The tank is screwed on via a 1 inch slip to thread adapter. I use a 1 inch srinkler valve as a pilot valve (This one's an Orbit Watermaster). I used a 1 1/4 inch coupler as the piston and filled it with epoxy. When I fill the gun with air, everything is airtight, but when I release the pilot volume, the gun's performance is less than impressive. It can still shoot a spud, but right now I'm getting better performance out of a cannon using a 1 inch sprinkler valve I'd post pics of it, but I'm way out of state for the moment. What should I do to improve performance? I was thinking that having air flow through a 1 inch hole twice(once to get out of the main tank on the gun, and again to get out of the cannon) was messing with performance, but since this is one of my first kind of successful piston valve air cannons I'm not really sure.

If its hard to move then it wont open at lower pressures... right?
If you could make the piston move easily but seal 100% with O rings you'd have a real winner Unfortunately if the piston moves easily it starts to leak air around itself.
You really got to balance adequate seal and ease of movement the hard way... Or a floating O ring design maybe?